# Seedling care: the first three weeks

> Seedlings need moderate light (about 150–300 µmol/m²/s, 18 h, a DLI of 10–19 mol/m²/day), warm and fairly humid air (23–27 °C / 73–81 °F, 65–75 % RH), and small, careful waterings. Most seedling losses come from overwatering, too little light (stretching) or damping-off, not from lack of nutrients. Start feeding lightly once the first two or three sets of true leaves have formed.

- Canonical: https://genetixbay.com/knowledge/germination/seedling-care.html
- Category: Germination · Level: Beginner
- Evidence: Supported · Status: Draft: awaiting expert review
- Stage: Seedling · Systems: Any system
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**Why is my seedling stretching and falling over?** A cannabis seedling that stretches into a long, thin, pale stem and flops over is usually getting too little light, or the light is too far away. Raise the light intensity gradually toward 150-300 [µmol/m²/s](https://genetixbay.com/knowledge/glossary/ppfd.md) for 18 hours, and at [transplant](https://genetixbay.com/knowledge/glossary/transplanting.md) bury the stretched stem deeper in soil or coco. If the stem is pinched and brown at the soil line instead, the cause is [damping-off](https://genetixbay.com/knowledge/glossary/damping-off.md).

**How much should I water seedlings?** Water cannabis seedlings in small amounts, in a ring around the stem, only when the plug or small pot feels lighter. Keep the surface just moist rather than wet, and use room-temperature water, since cold water slows growth and raises damping-off risk. Giving a big pot a big watering for a tiny root is a classic mistake that leaves the medium soggy and the seedling drooping.

**How far should the light be from seedlings?** The right distance from light to cannabis seedlings is whatever delivers about 150-300 µmol/m²/s at the canopy for 18 hours, a [DLI](https://genetixbay.com/knowledge/glossary/dli.md) of 10-19 mol/m²/day; the height depends on the fixture, so measure PPFD rather than guessing. Start lower, around 100-200 µmol/m²/s, as cotyledons open. Bleached tips or hard 'praying' leaves mean too close; long, pale stems mean too far.

**When should I start feeding seedlings?** Start feeding cannabis seedlings once two or three sets of true leaves have formed, at about quarter to half strength, an EC of 0.4-0.8 [mS/cm](https://genetixbay.com/knowledge/glossary/ms-cm.md) (200-400 ppm on the [500 scale](https://genetixbay.com/knowledge/glossary/ppm500.md)). Seedlings in inert media with no starter charge, such as coco, [rockwool](https://genetixbay.com/knowledge/glossary/rockwool.md) or hydro, need feed earlier. Yellowing cotyledons around days 10-14 are often normal, or a sign an inert medium needs a light feed.

**What humidity for cannabis seedlings?** Cannabis seedlings do best at about 65-75% [relative humidity](https://genetixbay.com/knowledge/glossary/relative-humidity.md) with air at 23-27 °C (73-81 °F), giving a leaf [VPD](https://genetixbay.com/knowledge/glossary/vpd.md) of roughly 0.4-0.8 kPa. A humidity dome helps in the first days, but vent it daily: a dome left closed for days invites mould and soft stems. These targets are grower practice rather than cannabis-specific research, so watch how your seedlings respond.

**Short answer.** Give seedlings **moderate light** (≈ **150–300 µmol/m²/s for 18 h**, a DLI of **10–19 mol/m²/day**), **warm, fairly humid air** (**23–27 °C / 73–81 °F, 65–75 % RH**, leaf VPD ~0.4–0.8 kPa) and **small waterings** in a ring around the stem. Hold off on feed until **2–3 sets of true leaves** unless the medium has no starter charge (coco, rockwool, hydro). Most losses are **[overwatering](https://genetixbay.com/knowledge/glossary/overwatering.md), stretching from low light, or damping-off**.

## Week by week

| Days | What you see | What to do |
|---|---|---|
| 0–5 | Hook emerges, cotyledons open | Low light (100–200 µmol/m²/s), dome vented daily, keep surface just moist |
| 5–10 | First true leaves (single or 3-finger) | 150–250 µmol/m²/s; water small amounts when the plug lightens |
| 10–21 | 2nd–4th nodes | 200–300+ µmol/m²/s; start ¼–½ strength feed (EC 0.4–0.8 mS/cm); transplant when roots fill the plug ([transplanting](https://genetixbay.com/knowledge/germination/transplanting.md)) |

## Numbers that matter

| Parameter | Target | Evidence |
|---|---|---|
| PPFD | 150–300 µmol/m²/s, 18 h | Grower practice; consistent with young-plant DLI research in other crops [1] |
| DLI | 10–19 mol/m²/day | Plugs below ~8 mol/m²/day are inferior in other crops [1] |
| Air temperature | 23–27 °C (73–81 °F) | Grower practice |
| RH | 65–75 % | Grower practice |
| Medium temperature | 21–25 °C (70–77 °F) | Warm roots grow fast; cold water raises damping-off risk [2] |
| First [feed EC](https://genetixbay.com/knowledge/glossary/input-ec.md) | 0.4–0.8 mS/cm (200–400 ppm500) | Grower practice; start at true leaves [2] |
| Water pH | Soil 6.2–6.8; coco 5.8–6.2; hydro/rockwool 5.5–6.0 | |

## Problems and fixes

| Symptom | Likely cause | Fix |
|---|---|---|
| Long, thin, pale stem, flops over | Light too weak or too far | Raise PPFD gradually; bury the stretched stem deeper at transplant (*Grower practice*) |
| Drooping, wet medium | Overwatering | Let it dry; smaller waterings; airier mix |
| Stem pinched and brown at the soil line, collapse | Damping-off | See [damping-off](https://genetixbay.com/knowledge/germination/damping-off.md) |
| Cotyledons yellow at day 10–14 | Normal as true leaves take over, or no nutrients in inert media | Start light feed |
| Leaves praying hard, bleached tips | Too much light/heat | Dim or raise light |
| Seed husk stuck on cotyledons | Dry surface | Mist, wait, then gently remove |

## Common mistakes

- Big pot + big watering for a tiny root.
- Dome left closed for days (mould, soft stems).
- Strong nutrients "to get it going".
- A cheap light at 1 m away.

## Evidence

Young-plant light responses are *Supported* in horticultural research on other crops [1]; cannabis veg responds to high light with compact growth [3]. Temperatures follow hemp [germination](https://genetixbay.com/knowledge/glossary/germination.md) data [4] and grower practice. Fertiliser timing follows extension guidance [2].

## Sources

1. Runkle E. The benefits of lighted plugs. Michigan State University Extension (plugs below DLI ~8 inferior; 10–14 mol/m²/day better roots and mass). https://www.canr.msu.edu/resources/the_benefits_of_lighted_plugs
2. University of Minnesota Extension. How to prevent seedling damping off (fertiliser after true leaves; avoid cold water). https://extension.umn.edu/solve-problem/how-prevent-seedling-damping
3. Moher M, Llewellyn D, Jones M, Zheng Y, 2021. High light intensities can be used to grow healthy and robust cannabis plants during the vegetative stage of indoor production. Preprints (not peer reviewed). https://doi.org/10.20944/preprints202104.0417.v1
4. Geneve RL, et al., 2022. Temperature limits for seed germination in industrial hemp. Crops 2(4):415–427. https://doi.org/10.3390/crops2040029